Furnace



Jgn. 31, 1939. H E. -r u AL 2,145,324

FURNACE Filed March 25, 1937 Patented Jan. 31, 1939 PATENT OFFICE FURNACE Henry E. Stalin, Maplewood, and Joseph G.

den, Kearny, N. 1., alslgnor's to Baker at 001m D N New Jersey Application M 25,

3 Claims.

This invention relates in general to furnaces and more particularly to so-called mume furnaces. The invention contemplates an electric, gas-fired or oil-fired furnace which shall have a wide range of use, for example, by dentists or dental technicians, jewelry manufacturers, laboratory workers, and others, for various purposes such as baking or glazing porcelain teeth, melting, annealing and so forth.

Furnaces of the general character described usually include a hollow body of refractory material to. receive the article or object to be heated and surrounded or contacted upon one or more walls, by a heater, for example, an electric heating coil, the hollow body being open at one or both ends, or being closed at both ends by removable doors or by a wall at one end and a door at the other.

A prime object of the present invention is to provide a furnace of this character embodying novel and improved features of construction whereby a substantially uniform distribution of heat throughout the heating chamber shall be ensured, and whereby a larger zone of substan- 26 tially uniform temperature within the furnace than heretofore possible shall be obtained without increase in the size of the furnace and without increase in the size or capacity of the heater.

Another object is to provide such a furnace embodying novel and improved features of construction whereby loss of heat by radiation shall be reduced to the minimum so that the required temperature may be maintained in the furnace efliciently .and economically, i. e., with a minimum of consumption of electricity where an electric heater is used, or with a minimum consumption of fuel where gas or oil heaters are used.

A further object is to provide an. electrically heated furnace of the character described embodying a novel and improved construction and combinationof parts wherein the differential between the temperature of the heater and the temperature in the heating chamber shall be small, so that the heater may operate for a longer period or may have a longer life by operating at lower temperatures than heretofore in maintaining the heretofore usual temperatures in the heating chamber, or a higher temperature than heretofore possible can be maintained in the heating chamber with the heater operating at a given heretofore usual temperature.

Other objects are to provide such a furnace which shall be simple, inexpensive, reliable and durable in construction; and to obtain-other N. J., a corporation of.

1937, Serial No. 132,924

results and advantages aswill be brought out by the'following description.

Referring to the accompanying drawing in which corresponding and like parts are designated throughout the several views by the same reference characters,

Figure 1 is a vertical longitudinal sectional view through an-electric furnace embodying our invention taken on the line l-i of Figure 2 and showing the mume in side elevation.

Figure 2 is a transverse sectional view on the line 2-2 of Figure 1, and

Figure 3 is an enlarged fragmentary longitudinal vertical sectional view through the furnace.

For the purpose of illustrating the principlesof the invention, we have shown it embodied in a furnace especially suitable for use by dentists or dental technicians, although obviously the invention may be embodied in furnaces for other uses and the details of structure of the furnace may be -modifled accordingly without departing from the spirit or scope of the invention.

Specifically describing the-illustrated embodiment of the invention, the furnace includes a hollow elongate body or muiiie I formed of suitable refractory material having side walls 2 and end walls 3 and 4. As shown, one end wall 4 is formed integral with the muille so as to close one end of the muflle, while the other end of the muflle is open and may be closed by a removable plug or cover 3 which is preferably formed of the same refractory material as the body I.

The side walls extend beyond the inner surfaces I and 6 of the end walls substantial distances as clearly shown in Figure 3, and preferably the end walls are relatively thick as compared with the side walls 2, the wall 4 preferably including heat insulating material within the side walls I8 which desirably is in powdered ofgranular form.

In this connection, it should be mentioned that instead of the flxed end wall 4, a second cover or plug like the cover 3 may be utilized.

The side walls and end walls thus form a heating chamber 1 between them, and the side walls are exposed to a heater 8 throughout the length of the heating chamber and such a distance beyond the inner surface of each end wall 3 and 4 that the heat from the heater is uniformly distributed throughout the heating chamber and the latter-is heated throughout to a uniform temperature. As shown in the drawing, the heater comprises a helical electrical resistance coil 8 which surrounds the exterior of the side walls lit with a plurality of convolutions disposed outwardlybeyond the inner surface of each end wall, and preferably, although not necessarily, the heating coil has a greater number of convolutions per unit of length of the coil at its ends beyond the inner surfaces of the end walls 3 and l as indicated at 9, than at the other portions of the coil. The heating coil may be formed of suitable material, for example platinum wire, and may be embedded in refractory cement or other suitable refractory material l0.

The distances which the ends of the side walls project beyond the inner surfaces of the end walls, or conversely, the distances at which the inner surfaces of the end walls are spaced from the ends of the side walls, may be varied, but are such that the side walls and end walls are heated so as to produce a substantially uniform temperature throughout the heating chamber 1. its shown, the inner surfaces 5 and 6 of the end walls are disposed inwardly of the ends of the side walls, a distance substantially equal to one fifth of the length of the side walls.

The mufle or body i may be mounted in any suitable manner, but as shown, has its ends set into rabbets or recesses 91 in end plates i 2 of a casing which also has a side wall l3 connecting the end plates, and the heater and muille are insulated in the casing by suitable heat insulating material 94, preferably in powdered or granular form. The end plates are connected together to hold the muille and side walls in position by tie bolts i5.

From the foregoing, it will be observed that the main feature of the invention is the location of the inner surfaces of the end walls substantial distances inwardly from the corresponding ends from the side walls, or conversely, the extension of the side walls substantial distances beyond the inner surfaces of the end walls, and the exposure of the end walls to the heater throughout the length of the heating chamber and distances beyond the inner surfaces of the end walls such that the heating chamber is heated throughout to a uniform temperature. In operation, the side walls coextensive with the heating chamber are directly heated by the convolutions of the heater between the inner surfaces of the end walls, while the end walls themselves are directly heated by the convolutions 9 of the heater which extend beyond the inner surfaces of the end walls. Consequently radiation of heat from the heating chamber through the end walls is reduced to the minimum and the inner surfaces of the end walls are heated to the same temperature as the hottest portion of the heating chamber. Accordingly there is no exchange of heat between the heating chamber and the end walls, and it is possible to maintain the desired temperature uniformly throughout the heating chamber efliciently and economically with a minimum consumption of electricity.

Also, the differential between the temperature of the heating coil 8 and the heating chamber is small so that the heater may operate for a longer period or may have a longer life by operating at lower temperatures than has heretofore been possible in maintaining the usual temperatures in the heating chamber, or a higher temperature than heretofore possible can be maintained in the heating chamber with the heating element operating at a heretofore given usual temperature. This is important, because the life of resistance elements depends upon the temperature to which they are subjected, deterioration increas g r pidly as the temperature increases, and the possibility of obtaining a given temperature in the heater. In fact, it is possible to utilize a shorter muflie, because with {the invention the whole heating chamber may be utilized due to its uniform temperature, whereas in the known types of furnaces only a portion of the heating chamber can be maintained at the required temperature so that it is impossible to utilizethe entire heating chamber for heating purposes.

Another advantage of the invention is that it permits the use of powdered insulation rather than solid insulating material which cracks and thus permits escape of heat and reduces the insulating value of the material. It is necessary, however, that with the powdered material, the ends of the muflle be directly connected to the end plates I2 of the casing, and these end plates 32 must be of material selected for its refractory and mechanical properties rather than heat insulating properties. With known constructions using powdered insulation much heat is lost from the muille to the said end plates, cooling the ends of the muille, and correspondingly reducing the temperature of the entire heating chamber. Our invention compensates for this difllculty in that the cooled ends are not exposed to the used portion of the furnace, and the end walls 3 and i are at substantially the temperature of the furnace itself rather than at the temperature of the cool ends. In this way a pronounced tem-- perature gradient in the furnace is avoided.

As hereinbefore indicated, the furnace has a wide range of use, but it is especially useful to dentists and dental technicians for glazing and baking porcelain teeth, especially in dentures such as bridges. Ordinarily, a dentist desires to glaze or bake a number of teeth, for example several separate teeth or several teeth in a bridge, and in order to properly fuse all of the porcelain, it is necessary to have a zone of uniform temperature sufficiently large to encompass the entire bridge work or all of the separate teeth. In known furnaces, frequently the zone of uniform heat distribution is too small to cover an entire bridge or a number of separate teeth, and as the result the heat applied to the bridge or teeth is not uniform and inaccurate results are obtained or excessive heating is resorted to.

Teeth are usually baked in temperatures ranging from 1300 C. to 1400" C., and it is well known that shades of teeth are affected by excessive heating as well as by exposure to drafts of fresh air. These results frequently occur with known types of furnaces due to the resort to excessive heating in an effort to overcome the lack of uniform correct temperature, and also due to the frequent opening of the furnace during heating for rearrangement of the teeth or bridge to obtain proper baking or glazing.

With our invention, a much larger zone of uniform temperature than has been heretofore possible and which is wholly adequate to properly fuse or bake the teeth in a bridge, is provided and without any increase in the size of the furnace or in the size or capacity of the heater, and a complete bridge or several teeth' can be fused or baked in one operation without opening of the furnace or rearrangement of the bridge or teeth during the baking operation.

In order to apprise the user of the temperature within the heating chamber 1, one end wall may be provided with an opening I! to receive a thermo-couple I8.

While we have specifically described only an electric heater, it should be understood that the invention is susceptible to use in gas-fired or oilfired furnaces.

Having thus described our invention, what we claim is:

1. In a furnace of the character described, the combination of a hollow refractory body having side walls and relatively thick end walls forming a heating chamber between them, and a heater outside said refractory body constructed and arranged to heat the exterior of said side walls throughout the length of said heating chamber and to heat said end walls substantial distances outwardly beyond the inner surfaces ofsaid end walls independently of the heat from said chamber and conduction from the side walls of said refractory body between said end walls so that the inner surfaces of said end walls shall be at approximately the same temperature as the hottest portion of said heating chamber, whereby to heat said heating chamber to a temperature uniform throughout the chamber and reduce radiation and conduction of heat from said chamber through said end walls.

2. In a furnace of the character described, the combination of a hollow refractory body having side walls and relatively thick end walls forming a heating chamber between them, and a heater outside said refractory body comprising a helical electrical heating coil surrounding said refractory body throughout the length of said heating chamber to heat the exterior of said walls throughout, the length of said heating chamber, said heating coil having a plurality of convolutions encircling said ends walls and extending substantial distances outwardly beyond the inner surfaces of said end walls to heat said end walls substantial distances outwardly beyond the inner surfaces of said walls independently of the heat from said chamber and independently of conduction from said side walls between said end walls so that the inner surfaces of said end walls shall be at approximately the same temperature as the hottest portion of the heating chamber, whereby to heat said heating chamber to a temperature uniform throughout the chamber and reduce radiation and conduction of heat from said chamber through said end walls.

3. A furnace of the character described comprising a hollow refractory body having side walls, one open end and an integral end wall disposed a substantial distance inwardly of the side walls, heat insulating material disposed within said side walls outwardly beyond said end wall, a plug of refractory material removably fitted in said open end with its inner surface disposed inwardly of the corresponding end of the side walls a distance substantially equal to the distance of said end wall from the corresponding ends of said side walls whereby to form a heating chamber within said side walls and between said end wall and the inner end of said plug, and a heater outside said refractory body constructed and arranged to heat the exterior of said side walls throughout the length of said heating chamber and to heat said end wall and said plug substantial distances outwardly beyond the inner surface of each of said end wall and said plug independently of heat from said chamber and conduction from said side walls between said end wall and said plug, so that the inner surface of said end wall and said plug shall be at approximately the same temperature as the hottest portion of said heating chamber, whereby said side walls, plug and end wall are heated to produce a uniform temperature throughout said heating chamber and reduce radiation and conduction of heat from said chamber through said plug and said end wall.

HENRY E. STAUSS. JOSEPH G. OGDEN. 

